An avidity test for measles IgG was developed and applied to the study of IgG immunoglobulin maturation kinetics in follow-up sera from 12 patients with known acute primary and convalescent measles and sera from blood donors. The avidity of the IgG anti-measles responses was measured using the 8 M urea elution technique, the results being expressed as the percentage ratio between the test readings for eluted and noneluted samples. The IgG avidity results were compared with those of indirect and mu-capture IgM enzyme immunoassays. This test was capable of detecting low-avidity antibodies at the acute phase of measles up to 7 weeks, and increasing avidity through immunosaturation during the convalescent phase. The avidity in these samples did not reach the level found in the samples of the blood donors under the follow-up time. Although a limited number of serum samples was examined, the results suggest that the measles IgG avidity test is a powerful tool for differentiating primary measles infection from the convalescent phase.